1. Field Of The Invention
This invention relates to a process for automatically controlling the velocity of strand sintering machines used in the sintering of iron-oxide bearing materials of fine particle size to maintain the burn-through point of the sintering mixture ahead of the discharge end of the machine and more particularly to such a process wherein the controlled variable used for this purpose is the temperature of the waste gases of the sintering operation taken at a point in the collecting pipe.
2. Prior Art
In the sintering operation of iron ores on strand sintering machines, the mixture to be sintered contains a fuel and the upper layer of the bed is ignited; a gas containing oxygen, air usually, is pulled through the mixture. The combustion zone then travels down vertically within the sintering mixture. The point where the combustion zone reaches either the fire bars or the grizzly layer is called the burn-through point. In processes where the sintered product is not cooled on the sintering strand, the burn-through point must lie close to the discharge end so as to utilize as fully as possible the sintering capacity of the machines and so as to avoid damage to the aggregates stemming from an early discharge and the discharge of unsintered material. In processes using partial cooling of the sinter on the sintering strand, the burn-through point must lie at a predetermined location which is held as constant as possible so as to realize constancy of operating conditions. To this end, it is necessary to adapt the travel speed of the strand sintering machines to the sintering velocity within the bed.
The ratio of the strand length L to the bed depth h is equal to the ratio of the appropriate strand travel speed V.sub.M to the vertical sintering velocity V.sub.S. Thus: L = const = V.sub.M .times. 1/V.sub.S .times. h. One can achieve automatic control by acting on the vertical sintering velocity, for example, by adjusting correspondingly the controls of the waste gas blower. But this would mean that the blower cannot always be used to its full capacity. For the same reason, the automatic control of the sintering velocity through choking of the waste gas flow is not taken into consideration. To achieve automatic control, one cannot resort to the bed depth since adjustments of the latter will result in variations of sinter properties, fuel requirements and sintering machine capacity. The only possibility left is thus to automatically control the velocity of the sintering machine. V.sub.S cannot be measured directly and one must rely upon a different criterion to follow the progress of the sintering operation.
Various processes are known for automatically controlling the travel speed (Cappel, Wendeborn "Sintering of Iron-Ores," Verlag Stahleisen m.b.H., Dusseldorf, 1973, pp. 251-253). When the automatic control is based upon the temperature distribution in the waste gases near the end of the strand, the tightness of the end face has an effect. When the last windbox or partial windbox is very short, a large air leakage at that spot can produce a shift in the peak value of the waste gas temperature; the temperature is then actually determined by the magnitude of the leakage at the end face. On the other hand, if the maximum temperature is not very pronounced, it is difficult to determine.
Furthermore, in this method of automatic control, the desired value of the location of the maximum temperature can be adjusted only within narrow limits; for larger limits, one must change the location of the temperature sensors; this is because the behavior of the waste gas temperature is assumed to be parabolic, but this is only valid in the vicinity of the maximum. For various ores, the temperature profile is flat anyway because the permeability of the bottom layers is very dependent on localized heat effects. In these cases, the maximum temperature of the waste gases is not a reliable criterion for the completion of sintering.
So as to avoid these difficulties, one has resorted also to another controlled variable: the temperature of the waste gases in the collector pipe at a point located before the inlet to the electric filter. The earlier the completion of the sintering process, the longer the sinter cools on the machine and the more heat is transferred from the sinter to the waste gas. A steady waste gas temperature corresponds to a steady travel of the ignited zone in the bed. The waste gas temperature is thus an appropriate measure of the progress of the process. This method of automatic control has, however, a large time constant.